KReePN

I think it sounds way better sealed off like that, less trunk rattles, and all of the energy goes into the cabin. Not to mention you can have more room for batteries and amps behind the box usually this way. Great stuff will do the trick. Here is a tip: use duct tape one one side of the edges around the box where you need to seal it up, I usually tape off the front of the box that faces in the cabin all around the top and sides, then I foam BEHIND the tape on the back side of the box. So once that dries you can peel the tape off of the front of the box and you will have a nice flush even edge all the way around the box. The tape also prevents the foam from expanding into the cabin and bulging out everywhere. Also don't forget to check your rear deck for holes that need to be sealed up too. I found some pretty big holes in my rear deck other than the speaker holes. I just dynamatted over those kinds of holes.

3 layers on all the seams DOES help. I use a combination of wood bracing and the glass matt. But I wouldn't rely on either one by itself. But 3 layers on all seams with some other bracing will stiffen it up and it will seal you up tight.